Studying On Microwave Plasma Technology Based On Coaxial Cavity

The microwave plasma has extensive prospect forground because of many advantages of high density of electrons reactive species,low temperature and easy to control,such as microwave ignition for automotive engine(microwave ignition),plasma metallurgy,plasma torch for chemical element detection,surface modificaiton and medical sterilization and disinfection.However,most of the microwave plasma equipment in the laboratory,is based on the microwave source of the CW magnetron with average power of kilowatt-hour),which is difficult to obtain large-scale application because of the complex structure and huge volume.In order to solve this technical problem,the microwave plasma application technology is studied in this paper to realize the miniaturization and lightweight of the microwave plasma equipment.The microwave coaxial transmission line is widely used because of its simple structure and small loss.A high electric field strength can be obtained at its open end with a very low microwave power based on the theory the coaxtal resonator.In order to cooperate with the increasingly mature microwave solid-state source technology,novel microwave devices with frequency modulation and tuning structure are designed in this paper based on the transmission line theory.On one hand,a higher electric field strength can be generated at the open end of the cavity,and the resonance frequency and the reflection coefficient can be flexibly adjusted.On the other hand,the full-automatic microwave plasma system can be constructed in cooperation with the microwave solid-state source.In this paper,the microwave spark plug device of the automobile engine and the small-sized microwave plasma torch device under the atmospheric pressure are both designed.The processing of the devices,the test system and the related experiment are completed,and the practicability of the microwave plasma technology is studied.The main work and innovation points of this paper are as follows:1.Microwave ignition:(a)Microwave spark plugs that are fed by two modes of electric and magnetic coupling are designed,and an equivalent circuit of the microwave spark plug is established;(b)the parameters affecting the performance of the microwave spark plug are studied,including the resonant frequency and the electric field strength,respectively.The parameters affecting the resonant frequency contains the thickness of the medium,the dielectric constant,the position of the feed,and the diameter of the central conductor;the parameters affecting the strength of the electric field include the structure of the discharge end of the center conductor and the outer conductor.c)The combustion efficiency and peak pressure in the combustion cylinder increase with the electric field strength.In addition,the plasma kernel generated in the combustion process is directly related to the central conductor structure:the larger the center conductor,the larger the plasma fire core,but the effect on the combustion propagation speed is limited;The experimental results of the electric coupling and the magnetic coupling spark plug are compared,and the electric coupling spark plug is combusted more fiercely than that with magnetic under the same electric field strength in some extent.2.Microwave plasma torch:A compact microwave plasma torch(CMPT)operating at 2.45 GHz with ultra-bandwidth of 30 MHz is developed in this paper.The frequency and reflection coefficient of CMPT can be adjusted respectively.Besides,this paper designs a fully automatic microwave plasma torch system for the first time.The effects of different gas flow rate and microwave powers on the CMPT discharge are carried out on this system.Finally,in order to confirm the properties of the CMPT,an experiment to modify the surface of polymethyl methacrylate(PMMA)is conducted.Experimental results indicat that:The larger the electric field strength,the more rough the surface of the treated PMMA,the smaller the contact angle between the water and the organic surface,and the more obvious the surface modification.3.The coupling between microwave and plasma has been a difficult problem to realize the large-scale application of microwave plasma because the load performance of the whole cavity body is changed after the plasma is generated,and the coupling of between the microwave source and the plasma is degraded,and as a result,the system efficiency is reduced.In this paper,the plasma is regarded as a dielectric with loss as well as plasma load,which is part of the whole plasma torch.The trend of frequency drift of the coaxial resonator is obtained through simulation and calculation,and the simulation results is consistent with the experimental data.The research results have laid a foundation for the miniaturization of microwave plasma devices.